Film is the most common originating material for television programmes. There has to be therefore a method of sequentially scanning frames of film to convert them into electrical signals for broadcast. Specialist machines, referred to as telecine machines exist for this purpose. Such machines have been in existence since the 1920's. Current examples of such machines include the "URSA" machine, made by Rank Cintel Ltd., Ware, England, and the FDL-90 Machine, manufactured by Broadcast Technology Systems (BTS) of Germany.
It is well known that when film is exposed to light and processed, the light causes a change in the density of the film. The characteristic relationship between the density of the film and the light required to produce that density is referred to as the "Gamma Curve". Essentially, it is found that this characteristic, when plotted on natural logarithmic axes of exposure versus density has a substantially linear portion. The gradient of this linear region is actually referred to as the "Gamma value". This is described well by standard texts in the subject, such as "The Reproduction of Colour in Photography, Printing and Television", By Dr. R. W. G. Hunt (Fountain Press, ISBN 085242356).
It is also well known that when a television monitor, or cathode ray tube, is used as a viewing device, there is a similar characteristic with the electrical voltage used to drive the device and the light emerging from the face of the cathode ray tube. This characteristic is also linear in a `Log-Log` space. The typical value of gamma for a television monitor is approximately 2.5.
Thus, where a cathode ray tube is used as the illuminant for a telecine scanning system, if this light source is modulated, this device has a first gamma relationship, whilst the film to be scanned also has a gamma relationship, the detector to collect and measure the light may have a gamma characteristic, and finally the television monitor that the operator is using to view the image will also have a fourth relationship. In practice the CRT light source would probably be "fully on" and thus the gamma characteristics of this device may not be relevant. In any event, in the process of scanning film for television there are up to four separate gamma characteristics that need to be corrected for, apart from the change of characteristic necessary to artistically "lighten" or "darken" material. Various arrangements have been proposed for the alteration of the overall gamma relationship of viewed images. One such technique was used successfully by Rank Cintel in its Mk III telecine, of which over 1000 units have been sold. This is illustrated in FIG. 1. This technique involves an electrical circuit with a "logging" characteristic, followed by a second circuit with a "multiplication" function, with variable operand, followed finally by a third circuit of exponential characteristic. Because of the age of the Mk III telecine all of the above is implemented in analogue electronics, which is widely known to be prone to electrical drift, noise, and "hum". The "drift", caused by amongst other things the slow changing of electrical characteristic over time has to be counteracted by frequent realignment of the electrical circuitry by skilled personnel. Whilst various measures can be taken to reduce the impact of noise, analogue electrical circuitry will always suffer from this problem.
Film is available in two main forms. The first of these is positive film, which corresponds to slide or transparency film in laymen's language. This is characterised by a dark portion of the film corresponding to a dark portion of the original scene. The second form is negative film, which corresponds to the film used domestically to produce photographic prints. This type is characterised by the light portions of the film corresponding to the dark portions of the original scene. It is expected that both types of film can be transferred on a telecine machine. The discussion below will be solely in terms of positive film, although all of the principles involved work equally as well with negative.
Advances in digital electronic circuitry made it possible by the late 1980's to build an almost entirely digital telecine system. One such machine is the Rank Cintel URSA, launched in 1989. The function of this machine is shown in FIG. 2. After the light is received by an electrical photomultiplier tube, the resulting electrical signal is digitised through a high precision analogue to digital (A/D) converter device. This signal is then passed through a digital look-up table (LUT), which has been preloaded with a logarithmic characteristic. After this table, a digital multiplier chip can be used to perform the gamma adjustment, where the operand can be user-selected to effect the gamma change required by the operator. This is finally followed by another look-up table to exponentiate the multiplied signal.
Whilst this circuitry, being digital, does not suffer from the disadvantages of drift, it is known and observed that it inherently produces visible noise in the darker portions of the picture. This is because the analog to digital converter chips inherently produce noise, as shown in FIG. 4A. The speed requirement for these chips, being typically 18 MHz or higher, together with the precision of quantisation required means that even the best available chips show a fairly even noise characteristic throughout their range. By this, it is meant that the amount of noise produced by these chips is not significantly different at high or low parts of the electrical signal level. The desired overall effect of the gamma change is to produce an increase in the differential (or contrast) of the shadow tones; a typical gamma curve is shown in FIG. 4B. This has the undesirable effect of enhancing the visibility of the noise and quantisation problems in these shadow tones as shown in FIG. 4C.
One method of reducing this effect would be to utilise lower noise analog to digital converters. This is not possible at the moment, as the best available chips are already utilised.